Turbine power plant control system

ABSTRACT

Thrust of a turbine type of power plant is controlled by closing the loop on fuel flow as a function of the difference between the desired thrust and a precalculated inferred thrust signal in combination with an optimization system for obtaining optimum thrust specific fuel consumption.

United States Patent Harner et a1.

Oct. 9, 1973 TURBINE POWER PLANT CONTROL SYSTEM Inventors: Kermit I. Harner, Windsor; Roy W.

Schneider, Ellington, both of Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

Filed: Oct. 29, 1970 Appl. No.2 85,045

U.S. Cl 235/l50.21, 60/3928, 235/l50.1 Int. Cl. F02c 9/04 Field of Search 235/l51.1, 150.21;

[56] References Cited UNITED STATES PATENTS 3,316,713 5/1967 Urban 60/3928 R OTHER PUBLICATIONS Self-organizing Control of Advanced Turbine Engines by Barron et al. Aug., 1969; Technical Abstract Bulletin No. 69-20, Oct. 15, 1969, No. AD 857-616, National Technical Information Service.

Primary Examiner-Eugene G. Botz Attorney-Norman Friedland [57] ABSTRACT Thrust of a turbine type of power plant is controlled by closing the loop on fuel flow as a function of the difference between the desired thrust and a precalculated inferred thrust signal in combination with an optimization system for obtaining optimum thrust specific fuel consumption.

4 Claims, 3 Drawing Figures TURBINE POWER PLANT CONTROL SYSTEM The invention herein described was made in the course of or under a contract thereunder with the Department of the Air Force.

BACKGROUND OF THE INVENTION This invention relates to a control system for turbotype of power plant of the type having at least one or both variable turbine inlet nozzles and variable area exhaust nozzles wherein thrust is obtained by closing the loop on fuel flow by comparing a precalculated inferred thrust signal with a desired thrust signal in combination with a self-organizing control system utilized for optimizing thrust specific fuel consumption at all thrust conditions by producing maximum cycle and propulsive efficiency.

The turbine type of power plant in current use today is generally of fixed geometry and that the only variable that affects control, performance and power of the engine is the quantity of fuel fed to the burner section. It is customary to provide in such a fuel control, sensors that monitor certain power plant operating conditions so as to schedule fuel flow to produce a desired thrust or power plant operating condition. Such heretofore known fuel controlsserve to schedule the steady state and acceleration and/or deceleration operating conditions in order to prevent overtemperature, surge, rich and lean blowouts. These controls are designed with a predetermined schedule reflecting the engines performance characteristics. While such schedules attempt to optimize fuel consumption and engine operating efficiency, they nonetheless are subjected to the preascertained performance criteria which may deteriorate after continuous usage of the power plant.

Since the advent of high performance turbine type of power plants which not only include the adjustable fuel flow but also include variable engine geometry such as variable turbine nozzles and variable exhaust nozzles, the problem of designing a fuel control that will achieve optimum fuel consumption is obviously more complex. To this end, this invention-relates to a system directed at achieving optimum thrust specific fuel consumption (TSFC) by controlling the thrust of the turbine power plant in a deterministic manner by adjusting fuel flow and to optimize TSFC in an adaptive" manner by adjusting engine geometry. While the term adaptive in the context of this specification is illustrated by the self organizing control system described in U. S. Pat. No. 3,460,096 granted to R. L. Barron on Aug. 5, 1969 it is not limited thereto. In this type'of system, power plant variables have characteristics which may be unknown and difficult to predict are continuously monitored during transient and steady state operating conditions to adjust fuel flow, turbine nozzle area, and exhaust nozzle area to experimentally calculate the performance of the power plant in order to zero in on the optimum TSFC. The self-organizing control system is utilized solely to control the geometry of the turbine power plant by providing a search for system parameter values during quasi steady state conditions for optimizing the engine s performance. The fundamental design of the controller incorporates the probability state variable search algorithm which is suitable for simultaneous adjustments in multiple parameters. The selforganizing control system includes a performance assessment unit which in part employs differentiation of difference techniques and logic for computing a binary performance value signal (V) which indicates if trends are favorable or adverse.

In the preferred embodiment of the self-organizing control system, a performance assessment unit receives measures of fuel flow and thrust changes to compute if TSFC is increasing or decreasing so as to make the determination independently of any thrust transients re-,.

sulting from power lever changes or environmental factors.

Correlation logic units of the self-organizing controller implement the parameter search under the guidance of the performance assessment unit for optimum engine areas or other parameter values being optimized. The basic design of the correlation logic units incorporates the probability state variable (PSV) search algorithm. This algorithm is suited for simultaneous adjustment for two or more parameters, providing the performance assessment unit has capabilities for resolving the very small performance differences which are obtained in the proximity of the optimum in the space of parameters. The correlation logic units are employed in alternating adjustments of the parameters for which the problem of performance resolution is greatly alleviated. In this instance a two state search is utilized in which simultaneous parameter adjustments are first variable being detected to find the sense of the next experiment, and all correlation logic units generate output changes simultaneously at a prescribed frequency or asynchronously. In the alternating search mode, the

correlation signal is detected without going through summation with a random variable and only one correlation logic unit generates an output change at a time, with each correlation logic unit taking its turn in a prescribed manner.

SUMMARY OF THE INVENTION The primary object of this invention is to provide a closed loop fuel control calculating an inferred thrust and comparing it with a desired thrust and closing the loop on fuel flow in combination with an adaptive type of self-organizing control system for optimizing TSFC.

Other features and advantages will be apparent from the specification and claims and from the accompanying drawings which illustrate an embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of the control system for turbine type power plant showing the preferred embodiment of this invention. f

FIG. 2 is a graphical illustration defining a typical curve showing the surge and temperature limit for variable turbine nozzle areas.

FIG. 3 is a graph illustrating a typical compressor map.

DESCRIPTION OF THE PREFERRED EMBODIMENT As used herein the Definition of Symbols are as follows: A, turbine nozzle area A,, N nominal turbine nozzle area A REF desired turbine nozzle area A, exhaust nozzle area A, R" desired exhaust nozzle area C, specific heat of gases k ratio of specific heats K change in exhaust nozzle area per unit output from SOC correlation logic unit K change in turbine nozzle area per unit output from SOC correlation logic unit K inferred thrust control derivative gain K, inferred thrust control integral gain K, inferred thrust control proportional gain M aircraft Mach number N rotor rpm N time rate of change of rotor rpm Pam ambient pressure P, compressor inlet pressure P compressor discharge pressure P, turbine inlet pressure P turbine discharge pressure q,= heating value of fuel T, compressor inlet temperature T, compressor discharge temperature T,, turbine inlet temperature T turbine discharge temperature W compressor airflow W,= main engine fuel flow W, R desired main engine fuel flow 17b burner efficiency 11c compressor efficiency "an exhaust nozzle adiabatic expansion efficiency 1 turbine efficiency 0 ratio of compressor inlet temperature to standard day temperature F, gross engine thrust F net engine thrust J mechanical equivalent of heat g acceleration due to gravity f, f f function of parameters AT time interval for updating SOC control outputs While in the heretofore known turbine type of power plant the problem of measuring the actual thrust is difficult, the problem is more complicated in the advanced type of engines where the engine geometry is varied. Unlike engines where the geometry is fixed measurement of compressor speed, for example, would be more indicative of thrust than would otherwise be in this type of advanced engine. To this end, the present invention utilizes a calculated inferred thrust by computing certain engine operating parameters.

To lend clarity to the understanding of this invention, the present portion of the disclosure is directed to a simplified calculation for obtaining an inferred thrust reference signal (all symbols are defined hereinabove).

A reasonable expression for gross thrust of a turbojet with variable turbine nozzle area is given by:

Gas flow, Wg, can be expressed as a function of the conditions entering the turbine as follows:

Wg A5 P./ Jr? f (In/P.)

where f1 1/ s) f )k1ik] U2 Substituting equation (3) into (1) yields 8 5 7fi (5/35) 1/ J 0,. L/gl l (Rim/ 1) k-l/k 1/2 (3) 1/ r) mat/P7) {I nT or, in its expanded form n =f( .-i 1/ 5) f (P /P turbine nozzle flow parameter In summary the jet engine thrust will be inferred from y the measurement of A P P-,/P,, and P /P as follows:

8 1f2 s, 1/ s)f( am/ 7) As noted from FIG. 1, the power lever signal 10 is compared with the thrust inference calculated signal 12 in a suitable electronic summer 14 for producing an error signal indicative of the difference between the desired and inferred thrust values which-signal is then sent through a proportional plus integral plus derivative controller. Such a controller is commonly known and described in detail in many textbooks including Principles of Servomechanisms by Gordon Brown and Donald Campbell published by John Wiley & Sons, Inc. January 1953, 5th Edition. Both the maximum and minimum permissible values of this controller are determined from the engine limit calculations (discussed later), where use of such controller limits are commonly understood in the art.

The signal generated by the proportional plus integral plus derivative controller, i.e., fuel flow reference (W is transmitted to the fuel control 16 which may take the form of any suitable fuel metering valve'to adjust fuel flow metered to the engine so as to provide the thrust necessary to minimize the error signal.

So as to prevent overtemperature and surge during acceleration, it is contemplated to limit the fuel flow rate by calculating the limits and adjust the W, n" signal accordingly. A better understanding of the limits necessary in this embodiment to prevent surge and maximum turbine inlet temperature can be had by referring to FIG. 2. FIG. 2 defines the surge and temperature limiting W,/P characteristics for an engine with variable turbine nozzle area. The temperature limiting curves 17 are for a 60 F compressor inlet temperature and 2,300 R turbine inlet temperature and the surge limits are shown as curves 19, being plotted for percentage of corrected compressor speed (N/ V While a rigorous mathematical computation would be necessary to show that surge and temperature can be ascertained by combining W,/P m N/ l 95, and A as control parameters, as will be obvious to one skilled in this art, the following computations will show the validity of the use of these parameters.

For choked turbine nozzles the following equation can be written u a Ms 5 1 Also, it is known that W,= W (1 W,/W,,)

Therefore, by rearrangement we can write P2 P4 (P2) (Pd (3) (ii l) M From the compressor equation the following can be defined Substituting equation (5) into equation (4a) and re-- arranging yields Since k (P /P C (1 W lW 17b, and q, are all nearly constant, it can. be seen that W,/ P m is uniquely defined for each point on the compressor map (see FIG. 3) provided A, is known. From equation 7) and the compressor map it is then possible to calculate W l P /T along the surge line for any value of A Withacombination of equation (3 and (7) it is also possible to define the limiting value of W;/P as a function of A,,, N andT tov provide the desired limiting turbine inlet temperature. X

FIG. 3 shows locus of constant corrected turbine inlet temperature (i.e., 73/0 for various (A /A and lines of constant W ll- V for 5 5 NOM' Obviously, to provide a theoreticaly-correct temperature limit it is necessary to define that limit as a function of N and T for a given turbine nozzle, area and then provide a biasing function as the area varies from its, nominal value. From. the data evaluated to date a reasonable. temperature limited value of W,/P' v can be defined from:

rs NOM 4v "1.1M 4 LIM AT NOM AREA 5 NOM designed to provide a reward (or penalty) binary signal from the computed improved (or degraded) performance assessment (PA). A reward signal causes the trend of perturbations in the future experiments to occur in the same direction that caused the reward. In a similar manner, a penalty signal causes the trend of perturbations in the future experiments to occur in the opposite direction. The continuous search for improved performance will result in the engine variables being adjusted, as required, to achieve the best performance. After having reached the best performance, the experiments will continue to occur but remain in the vicinity of the best performance.

It is obvious that the detailed mechanization of this concept can be done in various ways with a varying amount of complexity and sophistication.

In the preferred embodiment as can be seen from FIG. 1 and being one of the simplest mechanizations perturbs one variable at a time, then determine the resulting performance improvement (or degradation). After every variable, in turn, has been perturbed and a performance assessment for each has been determined, the sequence is repeated. The direction of the perturbation of each variable is determined by the preceding perturbation of that variable and the resulting computed performance assessment.

In FIG. I, the PA module 20 is designed to assess thrust specific fuel consumption, TSFC, where TSFC W,/Thrust. As would be obvious to one skilled in this art, it is evident that the PA module may be designed to provide a performance assessment of other parameters. The PA module receives signals from AF W,, N and computes a binary signal, SGN V, as follows:

Although a sensed AF is not normally available, a satisfactory substitute may be obtained from sensed aircraft acceleration and known aircraft mass in a conventional manner. During the time interval, AT the sensed AF changes from AF to AF Similarly sensed W, changes from W," to W, and N (being the instantaneous time derivative of sensed N) changes from N to N. It is evident that a AF AFNET 1 Will cause V to increase whereas W, Wf will cause V to decrease. The appropriately weighted sum of (AF AF and (W,-W,") will yield a V when thrust specific fuel consumption, W,/F- has improved (decreased), whereas V 0 if the thrust specific fuel consumption has degraded (increased). It is important to note that AT must be sufficiently long to permit the engine to be at steady state to obtain an accurate assessment if the V is based only on AF and W, as described above. The minimum permissible AT is predominantly determined by the engine rotor dynamic lag, such that AT must be at least 10 seconds at low power. The AT can be significantly reduced if the computation of V is modified to include a weighted value of(N N The weighted value of (N N is indicative of the amount of system performance being used to accelerate or decelerate the engine rotor that will ultimately appear as an increment in thrust. Hence, including (N N in the computation of V' results in V being equal to the steady state value before steady state conditions are actually achieved.

The synchronizer 24 establishes the time at which the signal, SGN V', is transmitted to each correlation logic unit (CL). The synchronizer in conjunction with the signal, SGN V determines SGN V and SGN V, by sampling SGN V at the appropriate time.

A correlation logic unit evaluates the preferred direction of the next perturbation, Au, based on the past Ap. (i.e., Au) and the resulting SGN V. Ap is a binary signal indicating the preferred direction of the next perturbation (+1 means to perturb +1; 1 means to perturb --l Thus, Au." is the value of Ap. determined in the preceding time interval. This can be mechanized by a simple multiplication of binary signals as shown:

F- 1 1) l i l 2 2) l 'z As can be seen in FIG. 1 there are two correlation logic units CLl for A, and CL2 for A There is always one correlation logic unit for each independent variable in the SOC. The synchronizer establishes the time of the next perturbation in Au. Au. and Au, are perturbed alternately at time intervals of AT The constants, K,, and K represented by blank box 26 26, scale the binary signals Au, and Apt respectively, to provide the desired perturbation magnitude in AA, and AA,,.

The SOC control of A, has provision for alternate control logic when engine limits have occurred. In particular, when overspeed occurs, the perturbations in AA, are always toward decreased A, to aid in eliminating overspeed and also permit the control to operate at the desired thrust and at maximum speed. The perturbations of A, in this alternate mode have a magnitude of AA,".

The up-down accumulators 28 28' collect and store the sum of all perturbations. The first order lags, included in smoothing filters 30 30' following the updown accumulators, are optional providing smoothing of the discrete steps in the up-down accumulator. These signals, A, up and A are the reference (or input) for the A, and A servos 32 34 respectively. The servo actuators adjust the engine geometry A, and A, in response to A, up and A, up in a conventional manner. It is recognized that such a SOC adjusting the engine operation to achieve the selected performance criteria (such as minimum TSFC as described herein) could cause the engine to violate one or more limits. This undesirable operation of the SOC is prevented by using the engine limit calculations to modify the SOC calculations as shown in FIG. 1 in either or both of the following ways.

If it is known, apriori, the direction of the control perturbation that will prevent the engine limit violation, then the engine limiter calculations provide the signal to the up-down accumulator 28' instead of the SOC 26'. For example, it is known that A] should always be decreased when engine speed exceeds its limit, therefore the limit calculation will perturb the up-down accumulator 28' toward decreasing AJ whenever the engine limit calculations indicated speed is too high.

Engine limits may exist in which it is uncertain how A] should be adjusted to avoid violating the limit. In this situation, the engine limit calculation provide the SGNV' signal instead of the PA providing the SGNV' signal to the binary multiplier 22. The SGV from the limit calculation is constructed using the same concepts as described for constructing a PA to minimize TSFC, where the choice of performance assessment minimizes engine limit violations.

A slightly more sophisticated SOC control, being a modification of the preceding SOC description can search out the best operating conditions faster when an SOC control has many independent variables. All the independent variables are perturbed simultaneously and the computed SGN V is the result of these combined perturbations. The computing mechanism for the SOC is not shown here for the sake of clarity, simplicity, and convenience, but for further details reference should be made to US. Pat. No. 3,460,096.

Only the correlation logic units and synchronizer described previously are modified; hence, only the operation of these units will be described here. The signal, SGN V, is continuously transmitted to every correlation logic unit, CL. The synchronizer establishes the time of the next perturbation in the Au. Generally the synchronizer is designed to cause a perturbation from every CL to occur simultaneously.

The CL first evaluates the preferred direction of the next perturbation, Au, based on the past A l. and the SGN V resulting from the simultaneous perturbations (in a manner such as described previously). This signal is smoothed in a filter such as a first order dynamic lag. A noise generator in each CL provides a random signal which is added to the output signal from this filter. The next perturbation, Au, is in the direction of the sign of this resulting signal. It can be seen that the Ap. from each CL tends to be in the direction yielding improved performance in the past several experiments. However, the non-deterministic manner used in computing each Au causes various combinations of Au perturbations to occur. Each CL'will tend toward providing a Ap. that has provided improved performance in the past several experiments. The overall effect is thatall independent variables tend to be adjusted simultaneously toward yielding the best performance.

It should be understood that the invention is not limited to the particular embodiments shown and described herein, but that various changes and modifications may be made without departing from the spirit or scope of this novel concept as defined by the following claims.

We claim:

1. A fuel control for a turbine type of power plant having a compressor, a burner, a turbine receiving the exhaust gases of said burner for driving the compressor, and having variable geometry where area varies, a source of fuel, means including a fuel metering means for metering fuel from said source to said burner, means responsive to a plurality of engine operating parameters for calculating an inferred thrust for any given engine operation, means for establishing a desired thrust, means responsive to the calculated inferred thrust and the desired thrust for controlling said fuel metering means, and a self-organizing control system responsive to a plurality of engine operating conditions for adjusting said variable geometry to obtain optimum specific fuel consumption.

2. A fuel control as claimed in claim 1 wherein said means for calculating an inferred thrust is responsive to turbine inlet pressure and turbine discharge pressure.

3. A fuel control as claimed in claim 2 wherein said means also is responsive to the area of the variable geometry.

4. A fuel control as claimed in claim 1 wherein said self-organizing control is responsive to the metered fuel flow, the net engine thrust and compressor speed.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,76%785 Dated October 9. 1.973

'Inventor(s) Kermit'l. Harner and Roy W. Schneider It is certified that error appears in the above-identified patent and that said Letters Patent are hereby correc ted as shown below:

Column 3, line 24, "N" should read Column 7 line 55, "N" should read Signed and sealed this 16th dag, of April 1971+.

(SEAL) Attesc:

' EDWARD I IJ LETCEMRJR. w 0. MARSHALL DANN Attestingg Officer I Commissioner of Patents Notice of Adverse Decision in Interference In Interference No. 100,444, involving Patent No. 3,764,785, I. Harner, R. W. Schneider, TURBINE POWER PLANT CONTROL SYSTEM, final judgment adverse to the patentees was rendered May 5, 1982, as to claims 1-4.

[Oflicial Gazette August I 7, 1982.] 

1. A fuel control for a turbine type of power plant having a compressor, a burner, a turbine receiving the exhaust gases of said burner for driving the compressor, and having variable geometry where area varies, a source of fuel, means including a fuel metering means for metering fuel from said source to said burner, means responsive to a plurality of engine operating parameters for calculating an inferred thrust for any given engine operation, means for establishing a desired thrust, means responsive to the calculated inferred thrust and the desired thrust for controlling said fuel metering means, and a selforganizing control system responsive to a plurality of engine operating conditions for adjusting said variable geometry to obtain optimum specific fuel consumption.
 2. A fuel control as claimed in claim 1 wherein said means for calculating an inferred thrust is responsive to turbine inlet pressure and turbine discharge pressure.
 3. A fuel control as claimed in claim 2 wherein said means also is responsive to the area of the variable geometry.
 4. A fuel control as claimed in claim 1 wherein said self-organizing control is responsive to the metered fuel flow, the net engine thrust and compressor speed. 